U.S. patent application number 14/163759 was filed with the patent office on 2014-09-18 for acoustic and optical illumination technique for underwater characterization of objects/environment.
The applicant listed for this patent is Kevin Kremeyer. Invention is credited to Kevin Kremeyer.
Application Number | 20140268107 14/163759 |
Document ID | / |
Family ID | 40563175 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140268107 |
Kind Code |
A1 |
Kremeyer; Kevin |
September 18, 2014 |
Acoustic and Optical Illumination Technique for Underwater
Characterization of Objects/Environment
Abstract
The invention provides a method for acoustically and optically
characterizing an immersed object of interest by generating a
serial plurality of acoustic and optical illumination pulses
through a liquid. In addition to the spectral analyses/imaging of
objects/environment made possible by the white-light illumination,
a target material can be ablated, generating an ionized plume to
spectrally identify the target's constituent atoms.
Inventors: |
Kremeyer; Kevin; (Tucson,
AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kremeyer; Kevin |
Tucson |
AZ |
US |
|
|
Family ID: |
40563175 |
Appl. No.: |
14/163759 |
Filed: |
January 24, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13526019 |
Jun 18, 2012 |
8675451 |
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14163759 |
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12289261 |
Oct 23, 2008 |
8203911 |
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13526019 |
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60960977 |
Oct 23, 2007 |
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Current U.S.
Class: |
356/72 |
Current CPC
Class: |
G01N 2021/1793 20130101;
G01N 2201/0697 20130101; G01N 21/718 20130101; G01N 21/6408
20130101; G01B 11/24 20130101; G01N 21/1702 20130101; G01N 29/2418
20130101; G01N 21/21 20130101; G01N 21/84 20130101 |
Class at
Publication: |
356/72 |
International
Class: |
G01N 21/84 20060101
G01N021/84; G01B 11/24 20060101 G01B011/24 |
Claims
1. A method for acoustically and optically characterizing an
immersed object of interest by generating a serial plurality of
acoustic and optical illumination sources in a liquid, the method
comprising: transmitting a serial plurality of laser-generated
optical pulses through the liquid; the optical pulses reaching
I.sub.LIB at optically specified locations, through pulse
compression and ionizing a liquid volume, thereby generating a
serial plurality of acoustic and optical illumination source
pulses, wherein the pulse compression is achieved through at least
one of a) optical group velocity dispersion induced longitudinal
compression of a frequency chirped optical pulse and b) transverse
self focusing via a nonlinear optical Kerr effect; and then
measuring and analysing the returned acoustic and optical
signals.
2. The method according to claim 1, wherein the liquid is
water.
3. The method according to claim 1, wherein the liquid is
seawater.
4. The method according to claim 1, wherein the pulse compression
includes both optical group velocity dispersion induced
longitudinal compression of a frequency chirped optical pulse and
transverse self focusing via a nonlinear optical Kerr effect.
5. The method according to claim 1, without lens focusing of the
optical pulses.
6. The method according to claim 1, with lens focusing of the
optical pulses.
7. The method according to claim 1, wherein the optical pulses have
a wavelength between 300 and 500 nanometers.
8. The method according to claim 1, wherein the optical pulses have
a wavelength less than 11 microns.
9. The method according to claim 1, wherein the optical pulses
travel through the liquid for distances of at least one meter.
10. The method according to claim 9, wherein the distances are
between 1 and 50 meters.
11. The method according to claim 1, wherein the optical pulses are
negatively chirped optical pulses.
12. The method according to claim 1, wherein the optical pulses are
negatively chirped optical pulses, and the liquid has a positive
optical group velocity dispersion parameter .beta..sub.2.
13. The method according to claim 1, wherein the optical pulses are
positively chirped optical pulses.
14. The method according to claim 1, wherein the optical pulses are
positively chirped optical pulses, and the liquid has a negative
optical group velocity dispersion parameter .beta..sub.2.
15. The method according to claim 1, wherein the optical pulses are
monochromatic optical pulses.
16. The method according to claim 1, wherein the optical pulses are
broadband optical pulses without chirp.
17. The method according to claim 1, wherein the optical pulses
have a wavelength varying effectively linearly with time.
18. The method according to claim 1, wherein the liquid is water,
including sea water, and the optical pulses are generated under
water.
19. The method according to claim 1, wherein the liquid is water,
including sea water, and the optical pulses are generated in air
and are transmitted into the water.
20. A method for acoustically and optically characterizing an
immersed object of interest by generating a serial plurality of
acoustic and optical illumination sources in a liquid, the method
comprising: transmitting a serial plurality of laser-generated
optical pulses through the liquid; the optical pulses reaching
I.sub.LIB at optically specified locations, through pulse
compression and ionizing a liquid volume, thereby generating a
serial plurality of acoustic and optical illumination source
pulses, wherein the pulse compression is achieved through at least
one of a) optical group velocity dispersion induced longitudinal
compression of a frequency chirped optical pulse and b) transverse
self focusing via a nonlinear optical Kerr effect; acoustically
locating the object of interest; and then measuring and analysing
the returned optical signals.
21. The method according to claim 20, wherein the optical signal
analysis consists of manipulating different polarization states of
the returned optical signals, in the form of comparing and
differencing them.
22. The method according to claim 20, wherein the optical signal
analysis consists of manipulating different spectral bands of the
returned white light optical signals, in the form of comparing and
adding them.
23. The method according to claim 20, wherein the optical signal
analysis consists of time-gating the images in order to compare and
add them in order to generate a 3D image of the target of
interest.
24. The method according to claim 20, wherein the optical signal
analysis consists of measuring the laser induced breakdown spectrum
of the object's laser-ionized material surface, ionized by one or
more shots at each ionized position to assess the object's surface
material composition at that position.
25. A method for characterizing a volume of liquid by generating a
serial plurality of acoustic and optical illumination sources in a
liquid, the method comprising: transmitting a serial plurality of
laser-generated optical pulses through the liquid; the optical
pulses reaching I.sub.LIB at optically specified locations, through
pulse compression and ionizing a liquid volume, thereby generating
a serial plurality of acoustic and optical illumination source
pulses, wherein the pulse compression is achieved through at least
one of a) optical group velocity dispersion induced longitudinal
compression of a frequency chirped optical pulse and b) transverse
self focusing via a nonlinear optical Kerr effect; rastering
throughout the volume; and then measuring and analysing the
returned line spectra of the laser induced breakdown to verify the
composition of the water and analysing the returned acoustic
signals to verify the location of the optical sources.
26. A method for characterizing a volume of liquid by generating a
serial plurality of acoustic and optical illumination sources in a
liquid, the method comprising: transmitting a serial plurality of
laser-generated optical pulses through the liquid; the optical
pulses reaching I.sub.LIB at optically specified locations, through
pulse compression and ionizing a liquid volume, thereby generating
a serial plurality of acoustic and optical illumination source
pulses, wherein the pulse compression is achieved through at least
one of a) optical group velocity dispersion induced longitudinal
compression of a frequency chirped optical pulse and b) transverse
self focusing via a nonlinear optical Kerr effect; rastering
throughout the volume; and then measuring and analysing the
returned absorption spectra of the conical emission of white light
to verify the composition of the water and analysing the returned
acoustic signals to verify the location of the optical sources.
27. A method for acoustically and optically imaging living tissue
by generating a serial plurality of acoustic and optical
illumination sources at the surface of the body, capable of
penetrating the tissue, allowing for optical and acoustic return
signals to be gated and read to reconstruct the tissue geometry.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/526,019, filed Jun. 18, 2012, which is a
continuation of U.S. patent application Ser. No. 12/289,261, filed
Oct. 23, 2008, now U.S. Pat. No. 8,203,911, which further claims
the benefit of priority from U.S. Provisional Patent Application
No. 60/960,977, filed Oct. 23, 2007. The foregoing related
applications, in their entirety, are incorporated herein by
reference.
[0002] The following patents and patent applications are each
incorporated herein by reference in their entirety: [0003] 1) U.S.
Pat. No. 6,527,221, which granted on Mar. 4, 2003, entitled
"Shockwave Modification, Method and Apparatus and System;" [0004]
2) U.S. Pat. No. 7,063,288, which granted on Jun. 20, 2006,
entitled "Shockwave Modification, Method and System;" [0005] 3)
U.S. Pat. No. 7,121,511, which granted on Oct. 17, 2006, entitled
"Shockwave Modification, Method and System;" [0006] 4) U.S. patent
application Ser. No. 11/288,425 filed on Nov. 29, 2005, now U.S.
Pat. No. 7,648,100, and entitled "Shockwave Modification, Method
and System;" [0007] 5) U.S. patent application Ser. No. 11/540,964
filed on Oct. 2, 2006, now U.S. Pat. No. 8,141,811, and entitled
"Shockwave Modification, Method and System;" and [0008] 6) U.S.
patent application Ser. No. 12/733,252, filed on Feb. 19, 2010,
which is the National Phase of International Patent Application No.
PCT/US2008/009885 filed on Aug. 20, 2008 and entitled
"Energy-Deposition Systems, Equipment and Methods for Modifying and
Controlling Shock Waves and Supersonic Flow."
BACKGROUND OF THE INVENTION
[0009] Self-focusing/compressing ultrashort pulse lasers are
employed to generate acoustic and light sources that can
acoustically, optically, and spectrally characterize underwater
objects and environments and also be used to transmit data. The
disclosed technology comprises a technique to characterize undersea
objects and environments in ways that have never before been
possible. The technique combines very short acoustic and optical
pulses which provide broad-band "illumination" over the full white
light optical spectrum, as well as over a very broad acoustic
range, up to several Megahertz. In addition to the spectral
analyses/imaging of objects/environment made possible by
white-light illumination, a target material can be ablated,
generating an ionized plume to spectrally identify the target's
constituent atoms. This approach combines a number of cutting edge
technologies, each of which has been demonstrated to some extent in
different environments or with different laser pulses. As a result,
although the technologies are complex and involve extension into
new regimes, each element is grounded in past experiments, and it
is their combination here and application in new environments that
constitutes the primary advance.
[0010] Characterization of the environment and objects is often
performed using acoustic-imaging techniques which involve
"illuminating" the targeted scenes with large amounts of acoustic
energy, centered around relatively low frequencies, with relatively
narrow bandwidths. Optical characterization can also be performed,
but again typically requires large amounts of illumination energy,
especially considering the stronger attenuation in the ocean of
optical energy than acoustic energy. There are several problems
with illuminating the undersea environment with large amounts of
energy. From a militarily tactical perspective, this practice
generates a strong signature, advertising the illuminator's
presence and allowing adversaries to much more easily detect, and
then evade and/or find them. From an environmental perspective,
depositing large amounts of energy into the ocean can damage the
sea life/environment, resulting in unwanted effects and
repercussions. A further technological advantage is that the
broad-band, short acoustic and optical pulses will allow much
greater resolution than the relatively long and narrow-band
illumination pulses currently employed. Rastering high
repetition-rate pulses to form a spatial array will allow for yet
greater acoustic resolution, while time-gating the measured return
signals (acoustic and/or optical) will provide much greater spatial
resolution and penetration through turbid waters.
[0011] Combining ultrashort pulse lasers and short-gate imaging,
Zevallos et al. have demonstrated the ability to resolve images
through murky scattering environments, which were formerly
completely impenetrable using any other optical means (Manuel E.
Zevallos L., S. K. Gayen, M. Alrubaiee, and R. R. Alfano,
"Time-gated backscattered ballistic light imaging of objects in
turbid water", Appl. Phys. Lett. 86, 011115 (2005)). Employment of
femtosecond continuums adds a spectral element, not only allowing
additional diagnostics (by seeing the spectrally-resolved return
signals), but also ensures the presence of the least attenuated
wavelength(s) for any given environment, exceeding those of
state-of-the-art underwater LIDAR systems, which employ longer,
monochromatic laser pulses. The shorter acoustic and optical pulse
widths can enable increases in resolution of up to three orders of
magnitude, and the increased penetration capability and time-gated
imaging is anticipated to increase range by at least one to two
orders of magnitude. The materials discrimination capabilities,
made possible by laser-induced breakdown spectroscopy (A. Michel,
M. Lawerence-Snyder, S. M. Angel, A. D. Chave "Oceanic Applications
of Laser Induced Breakdown Spectroscopy: Laboratory Validation",
2005 IEEE/MTS Annual Meeting); comparison of differently-filtered
images, and bio-mimetic signal processing of broadband acoustic
return signals, is a yet further benefit, which will allow an
entirely new capability in target
identification/discrimination.
[0012] Mullen et al. (L. J. Mullen, P. R. Herczfeld, and V. M.
Contarino, IEEE Trans. Microwave Theory Tech. 44, 2703 (1996)) and
Strand et al. (M. P. Strand, in Detection Technologies for Mines
and Minelike Targets, edited by A. C. Dubey, I. Cindrich, J. M.
Ralston, and K. Rigano [Proc. SPIE 2496, 487 (1995)]) have clearly
articulated the need for new technologies to increase range and
resolution in performing shallow-water surveying and underwater
mine detection in turbid waters. The effectiveness of the employed
techniques determines which waters can and cannot be
mapped/characterized in advance, and once in a given environment,
the ability to detect and characterize dangers ahead of a craft
places constraints on speed and the ability to maneuver. Positive
identification of obstacles is furthermore required to eliminate
the need to treat debris the same way one treats a mine Beyond
operation in the field, the need for these capabilities is further
required to help counter the increasing asymmetric threat coming
from terrorist activities both abroad and at home. If the proposed
approach increases range, resolution, and certainty by one to three
orders of magnitude, it will allow a vehicle to proceed more
quickly by the same amount when probing for dangers. For example, a
10-fold increase will allow an increase from 3 knots to 30 knots,
which is operationally very significant. These capabilities are of
great interest to both the United States Coast Guard and the United
States Navy, as well as to merchant, commercial, and private vessel
operators.
[0013] The disclosed illumination approach addresses a number of
current operational problems faced by the military, including
characterization of the littoral environment and identification of
mines, unexploded ordnance, and environmental impact/effects. This
optical and acoustic characterization is of particular importance
in submarine situational awareness, since it will provide high
quality imaging and enhancement of collision avoidance capability.
Targeted high-resolution acoustic imagery is often difficult to
obtain in a complex environment, and optical characterization can
be difficult or impossible to obtain with monochromatic sources and
especially in turbid waters. A direct benefit of the disclosed
technique will be enhanced remote sensing/detection of ordnance in
near-shore locations. A further benefit of the spectral aspect of
the technology is its potential use as a tool by both the US Navy
and Coast Guard for pollution/HAZMAT prevention and response
decision. Not only will the spectral analysis ability help in
material identification, but it can also be coupled to software and
databases for automation of this task.
[0014] The claimed technology is a multifaceted tool to address
several problems, which are currently approached in a number of
ways. Identification of chemicals is often done through water
sampling and chemical analysis, which is a time-consuming and
cumbersome process. Identification of a target material is often
done through visual imagery and assessment of the material's
acoustic impedance-mismatch with water. This can be an uncertain
process and is susceptible to deception techniques. In many cases,
positive identification requires close proximity to the target,
with the best diagnosis involving the deployment of a diver.
However, close proximity of divers and assets is undesirable when
assessing the nature of a given target object. Maintaining a large
stand-off distance and illuminating the object with white light and
acoustic energy currently requires a large flash and loud ping.
However, the nature of maritime scatterers tends to diffuse this
input energy and "blur" the final results.
[0015] Another approach to characterizing targets and the maritime
environment is to obtain spectral information. Performing these
measurements on the water column allows the assessment of its
chemical content, however conventional means to acquire spectral
information require either direct sampling or a remote white light
source that points through the targeted medium to a spectrometer.
These approaches can be time-consuming and risky because of the
proximity of either a vehicle and/or tether. Direct spectral
analysis of a solid target using conventional techniques also
requires physical contact with the target. Again, this is
inherently dangerous, and the identified concerns in these prior
capabilities are obviated through the disclosed method.
[0016] The coupling of electromagnetic/optical probing techniques
to acoustic signals presents the potential to decouple the observer
from the water, which has sparked significant interest and
extensive research. One current worldwide effort is to increase the
Maritime Domain Awareness. An important goal for this program is to
develop passive acoustic sensors which can be liberally deployed,
measure a wide range of signals, and require little to no power or
maintenance. Employing optical techniques to probe the acoustic
environment represents an approach with many benefits, in that
signals can be emitted and measured from outside the water, without
depending on assets in the water. Two of the key organizations
involved in this effort are the U.S. Coast Guard (USCG) and the
National Oceanic and Atmospheric Administration (NOAA). Their
interest in this application is further reason to pursue the
investigation of coupled optical and acoustic measurements.
[0017] The ability to remotely create an acoustic signal in the
water has been investigated using both continuous wave and pulsed
lasers by a number of Department of Defense (DoD) agencies. The
Naval Undersea Warfare Center--Newport Division has worked on
developing laser acoustic source and detection schemes at the ocean
surface in order to communicate with undersea vehicles. Their
approach allows for minimal air propagation, requires lens
focusing, and takes advantage of neither underwater optical
propagation nor the remote optical compression enabled by
ultrashort pulse lasers. Their modulated continuous wave laser
acoustic source arrays at the ocean surface also do not benefit
from underwater laser propagation. The methods furthermore yield
undesirably low efficiencies and weak acoustic signals because the
mechanism to generate acoustic signals when using low laser
intensities involves heating instead of optical breakdown. To
investigate the benefits of optical breakdown, the Naval Research
Laboratory (NRL) has had several groups investigate this technique
to generate acoustic signals using high-intensity pulsed lasers.
Certain groups have investigated short-pulse lasers (nanoseconds)
with pulse energies exceeding 100 Joules (J), while other groups
have investigated ultrashort laser pulses (sub-picosecond pulses)
with milliJoules (mJ) of energy per pulse. Their acoustic results
are sufficiently pertinent that we have included some of them in
the descriptions of section B. They have recently received a (U.S.
Pat. No. 7,260,023) for the generation of acoustic signatures using
non-linear self-focusing, with an ongoing application
(20060096802).
[0018] S. V. Egerev describes development of noncontact laser
acoustic sources in "In Search of a Noncontact Underwater Acoustic
Source", Acoustical Physics, vol. 49, issue 1, pages 51-61, 2003. A
laser-based ultrasonic and hypersonic sound generator is discussed
in U.S. Pat. No. 3,392,368 to Brewer et al. Laser induced electric
breakdown in water is discussed by C. A. Sacchi in the Journal of
the Optical Society of America B, Vol. 8, No. 2, February 1991,
pages 337-345. P. K. Kennedy discusses laser induced breakdown
thresholds in ocular and aqueous media in IEEE Journal of Quantum
Mechanics, Vol. 31, No. 12, December 1995, pages 2241-2249 and
2250-2257. A. Vogel and S. Busch discuss shock wave emission and
cavitation generation by picosecond and nanosecond optical
breakdown in water in J. Acoustical Society of America, Vol. 100,
Issue 1, July 1996, pages 148-165.
[0019] T. G. Jones, J. Grun, L. D. Bibee, C. Manka, A. Landsberg,
and D. Tam discuss laser-generated shocks and bubbles as
laboratory-scale models of underwater explosions in Shock and
Vibration, IOP Press, Vol. 10, pages 147-157, 2003.
[0020] P. Sprangle, J. R. Penano, and B. Hafizi discuss propagation
of intense short laser pulses in the atmosphere in Physical Review
E, Vol. 66, 2002, pages 046418-1-046418-21. The optical Kerr
effect, a non-linear change in the refractive effect at high
intensity, is discussed by Siegman, Lasers, pages 375-386,
1986.
BRIEF SUMMARY OF THE INVENTION
[0021] A method employing ultrashort pulse lasers to generate
acoustic and optical sources to characterize the marine environment
and underwater targets allows far more benefits than simply the
creation of an acoustic pulse. The disclosed technology combines
optical interrogation techniques with the previously-explored
acoustic measurement. The optical techniques include the ability of
ultrashort laser pulses to generate a white light continuum for
imaging, as well as generate a spectral signature from
laser-induced spectroscopy (LIBS) at the target surface. The
optical measurements can also be time-gated to select only the
return pulse from the desired target. This requires knowledge of
the distance to the target, which can be determined from the
acoustic echo-time. Any one of these approaches has potentially
great benefits, and their combination will provide yet greater
flexibility in developing applications and solutions to existing
problems. The disclosed system simultaneously takes advantage of as
many of the ultrashort pulse laser effects as possible. The very
short and broad-band optical and acoustic pulses we generate will
allow much higher resolutions to be achieved in both optical and
acoustic imaging over conventional methods. Our goal is to couple
these abilities, along with the related spectral
identification/characterization techniques, to develop a powerful
new tool, capable of high-fidelity acoustic and optical imaging
through turbid water, as well as remote material
identification.
[0022] Embodiments entailing generating an acoustic and optical
source in a liquid, the method comprising: transmitting an optical
pulse through the liquid; the optical pulse reaching and/or
exceeding the intensity required for laser induced breakdown of the
liquid (I.sub.LIB) through pulse compression and ionizing a liquid
volume, thereby generating an acoustic pulse, wherein the pulse
compression is achieved through at least one of optical group
velocity dispersion induced longitudinal compression of a frequency
chirped optical pulse and transverse self focusing via a nonlinear
optical Kerr effect.
[0023] Another embodiment of the invention is directed to a method
for generating a series of acoustic and optical illumination
sources in a liquid, the method comprising: generating and
transmitting a plurality of optical pulses through the liquid; the
optical pulses reaching I.sub.LIB through pulse compression and
ionizing a liquid volume, thereby generating a plurality of
acoustic pulses and/or sources and optical illumination sources,
wherein the pulse compression is achieved through at least one of
optical group velocity dispersion induced longitudinal compression
of a frequency chirped optical pulse and transverse self focusing
via a nonlinear optical Kerr effect; and steering each optical
pulse with a reflective surface.
[0024] Pulse compression can include both optical group velocity
dispersion induced longitudinal compression of a frequency chirped
optical pulse and transverse self focusing via a nonlinear optical
Kerr effect.
[0025] The liquid can have a positive or negative optical group
velocity dispersion parameter .beta.2, and the optical pulse can
have a corresponding negative or positive frequency chirp. In some
embodiments, the optical pulse has a wavelength varying with time,
including, but not limited to varying linearly in time. In other
embodiments, the optical pulse can be a monochromatic optical pulse
or a broadband optical pulse without chirp.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1A is a schematic view of a method for remotely
generating an acoustic and optical illumination source according to
an embodiment of the invention.
[0027] FIG. 1B illustrates a negatively chirped optical pulse
before propagation.
[0028] FIG. 1C illustrates the optical pulse of FIG. 1B after
propagation and longitudinal compression.
[0029] FIG. 2 shows a typical intensity profile of a laser
generated optical pulse before propagation through a liquid.
[0030] FIG. 3 shows the calculated intensity of a laser generated
optical pulse of FIG. 2 after a computer simulation of propagation
through water, according to an embodiment of the invention.
[0031] FIGS. 4 and 5 illustrate the calculated amount of pulse
compression when propagating through water a distance approximately
twice the attenuation length, according to an embodiment of the
invention.
[0032] FIGS. 6 and 7 illustrate computer simulations showing the
effect of pulse compression on the pulse duration, spot size, and
pulse intensity.
[0033] FIG. 8 illustrates a system including a repetitively pulsed
laser with a moving mirror for generating multiple acoustic pulses
in different locations, in accordance with one or more embodiments
of the invention. In addition to the non-collinear placement of the
acoustic/optical placement, the moving mirror may be held
stationary or excluded if desired, in order to place the generated
acoustic/optical sources in-line between the laser and/or mirror
and the object of interest to be characterized.
[0034] FIG. 9 illustrates a system in which a laser and acoustic
detector locate and image an underwater object of interest
including, but not limited to mines, unexploded ordnance, coral
reefs, ocean life, submarines, features of the ocean floor and
subsurface features, through acoustic/optical source/pulse
generation, in accordance with one or more embodiments of the
invention. In addition to the non-collinear placement of the
acoustic/optical placement, the generated acoustic/optical sources
may also be in-line between the laser and object to be
characterized.
[0035] FIG. 10 illustrates a system in which acoustic/optical
illumination sources/pulses are formed at expected positions and
times, allowing an undersea vehicle to determine its position
through triangulation, according to one or more embodiments of the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] The method for remotely generating a combined acoustic and
optical illumination source in water or another liquid having
optical group velocity dispersion occurs through multiple
mechanisms. The acoustic source is generated through a
photo-acoustic sound generation technique, capable of generating an
acoustic pulse at a predetermined remote underwater location many
meters from the laser source. The remote acoustic generation occurs
in two phases: 1) underwater laser pulse propagation and
compression using some combination of group velocity
dispersion-induced longitudinal compression, and transverse
focusing due to the nonlinear refractive index of the liquid, and
2) laser-induced breakdown, heating and vaporization of a liquid
volume, followed by rapid expansion and generation of a shock wave
that can serve as a useful acoustic pulse. The concurrently
generated optical illumination source arises through several
potential mechanisms, including but not limited to: incoherent line
emission and scattering from the generated plasma; emission of
white light, such as conical emission and/or emission from
self-phase modulation, either with or without significant plasma
generation; coherent light passing through and/or refracting
through the generated plasma; coherent and incoherent light shifted
from the original laser wavelength also propagating through,
refracting through, or scattering through/from the generated
plasma.
[0037] FIGS. 1A-1C illustrate schematically the system and method
for remotely generating an acoustic and optical illumination source
according to one or more embodiments of the invention. A laser
source 10 generates an optical pulse 20. The optical pulse 20
travels a distance in the water or other liquid having group
velocity dispersion, characterized by the parameter .beta..sub.2.
The optical pulse is transversely and/or longitudinally compressed
as it travels, until the intensity of the pulse is sufficient to
cause laser induced breakdown. The propagation paths of the outer
edges of the optical pulse are depicted by two solid lines 12 and
14, showing potential non-linear Kerr self-focusing of the pulse.
The pulse can simultaneously undergo longitudinal compression due
to group velocity dispersion.
[0038] The wavelength of the laser is preferably selected to be a
wavelength having a low attenuation in the water or other desired
liquid, as attenuation can be a strong function of the
wavelength.lamda.. Attenuation of light in water can be
characterized by an attenuation length L.sub.atten, with the beam
intensity decreasing with propagation distance z according to
I(z)=I(0) exp (-z/L.sub.atten). In pure water, maximum transmission
(and minimum absorption) occurs generally in a wavelength range of
300-500 nanometers, with a maximum attenuation length in this range
of approximately 50 meters. For sea water, the attenuation length,
L.sub.atten, is a function of impurity concentrations, with typical
values of 5 to 10 meters. The global average L.sub.atten is
approximately 4 meters, and for relatively clear ocean water
L.sub.atten can be 10 meters or greater. For embodiments in which
the maximum energy is required at the acoustic source, the
propagation path length should be selected to be less than
L.sub.atten. For applications requiring lower energy, the total
underwater propagation path can be a few times greater than the
attenuation length.
[0039] For optimal transmission in water, the wavelength .lamda. of
the optical pulse can be between about 300 nm and 500 nm, or 260 nm
to 650 nm. In one embodiment, a commercially available broadband
ultrashort pulse laser of wavelength range somewhere between
roughly 740-810 nm generates pulses of duration between roughly 20
to 120 femtoseconds, and a frequency doubling crystal converts a
portion of the energy to a wavelength range somewhere roughly
between 370-405 nm. In another embodiment, an Nd-doped laser
produces pulses of duration between roughly 2-10 nanoseconds at a
wavelength in the range of 1050-1070 nanometers, and a frequency
doubler converts a portion of the energy to a 525-535 nm
wavelength.
[0040] Although not thusly limited, the pulse 20 is preferably
frequency chirped, with its wavelength and frequency being a
function of time. For liquids such as water, where .beta..sub.2 is
positive, the pulse must be negatively frequency chirped, so that
the pulse has a shorter wavelength at the head of the pulse and a
longer wavelength at the end of the pulse. Such a negatively
chirped pulse in a liquid having a positive .beta..sub.2 will
compress longitudinally as it propagates. Although not thusly
limited, for a liquid with linear group velocity dispersion, the
wavelength of the pulse should be a linear function of time to
achieve optimal longitudinal pulse compression.
[0041] The chirped pulse can be generated by optical grating-based
dispersion such as that occurring in a chirped pulse amplifier
laser, or by any suitable dispersion method.
[0042] Longitudinal compression of the optical pulse as it travels
through the liquid relies on the group velocity dispersion (GVD)
parameter of the liquid, .beta..sub.2. The GVD parameter,
.beta..sub.2, is proportional to the rate of change of group
velocity of light with wavelength
.differential..nu..sub.g/.differential..lamda. over a range of
frequencies, and is positive for water. Therefore, in water, the
light with a longer wavelength travels faster than light with a
shorter wavelength. For an optical pulse with negative frequency
chirp, the initial shorter wavelength portions of the optical pulse
travel slower through the liquid than the later, longer wavelength
portions. The pulses are thus longitudinally compressed, so the
pulse duration is shortened as the optical pulses travel through
the liquid. For a negatively chirped pulse in which the wavelength
of the pulse is a linear function of time in a medium with linear
GVD, the propagation distance L.sub.GVD needed to produce maximum
longitudinal pulse compression is approximately equal to
T(0)/.beta..sub.2.delta..omega., where T(0) is the initial pulse
duration and .delta..omega. is the frequency bandwidth. Control and
variation of the initial pulse length T(0) and/or the laser
bandwidth .delta..omega. provides control of the of the
longitudinal compression range.
[0043] As the pulse duration is shortened through the longitudinal
compression, the intensity of the pulse increases, as illustrated
in FIG. 1C.
[0044] Transverse compression of the pulse occurs generally when
the optical intensity of the pulse is sufficiently high to induce
nonlinear optical effects (for example, nonlinear self focusing or
NSF). The threshold intensity above which nonlinear optical effects
are induced is represented by
P.sub.NSF=.lamda..sup.2/2.pi.n.sub.On.sub.2, where n.sub.0 is the
linear index of refraction and n.sub.2 is the nonlinear index of
refraction, and an approximation of the overall index of refraction
to the lowest order in the pulse intensity is n=n.sub.0+n.sub.2I.
As an example, for light with a wavelength of 400 nm, P.sub.NSF is
on the order of 1 megawatt in water.
[0045] In light with high intensities (light with power above
P.sub.NSF), the intensity excites a significant nonlinear response
of the refractive index (the Kerr optical effect). The nonlinear
refractive index induces a transverse nonuniformity of the beam or
pulse, with a higher index of refraction seen in the center of the
beam compared to the transverse outer portions of the beam or
pulse, resulting in self-focusing of the beam or pulse.
[0046] A characteristic distance for the transverse nonlinear self
focusing is approximately
L NSF = z R P ( z ) P NSF - 1 , L . sub . NSF = z . sub . R / (
sqrt ( P ( z ) - P . sub . NSF ) - 1 ) ##EQU00001##
where z.sub.R is the Raleigh range and is equal to
z.sub.R=n.sub.0.pi.R.sup.2/.lamda., and R is the initial beam
radius. For optimal pulse compression in a given medium, L.sub.NSF
is therefore determined by P(0) and R, which should be set such
that L.sub.NSF=L.sub.GVD and longitudinal and transverse
compression occur simultaneously.
[0047] In a preferred embodiment, the initial beam size and initial
beam power P(0) are selected so the P.sub.NSF threshold will be
exceeded during propagation, thereby inducing non-linear effects,
and the transverse self focusing and longitudinal compression occur
simultaneously. Simultaneous longitudinal and transverse optical
pulse compression can then occur at a chosen distance, which can be
less than, equal to, or greater than the optical attenuation
length.
[0048] Referring again to FIG. 1A, in an initial portion 40 of the
path length L, GVD longitudinal compression increases the intensity
of the negatively chirped optical pulse, triggering a non-linear
transverse self-focusing effect. The intensity at any point z along
the propagation direction can be represented as
I ( z ) = R 2 ( 0 ) T ( 0 ) R 2 ( z ) T ( z ) I ( 0 ) exp ( z L
atten ) . . I ( z ) = ( R . sup .2 ( 0 ) T ( 0 ) / R . sup .2 ( z )
T ( z ) ) I ( 0 ) exp ( z / L . sub . atten ) ##EQU00002##
In a second portion 50 of the path length, both longitudinal and
transverse compression occur, further increasing the intensity of
the light energy in the pulse. Convergence during nonlinear self
focusing extends over a distance of only a few centimeters in a
preferred embodiment.
[0049] Note that FIGS. 1A-1C are not to scale, and the transverse
width is exaggerated to illustrate the NSF effect.
[0050] FIGS. 2 and 3 illustrate the results of a computer
simulation of underwater laser pulse propagation, to show the
effects of group velocity dispersion and nonlinear self focusing on
an optical pulse. In this example, the laser is a commercially
available frequency doubled chirped pulse amplified ultrashort
pulse laser, and the optical pulse has a wavelength of 400 nm, an
initial pulse duration T(0) of 100 picoseconds, an initial pulse
energy E(0) of 0.55 mJ, an initial beam radius R(0) of 0.29 cm, and
a frequency bandwidth |.delta..omega./.omega.| of 2.5%. The medium
through which the optical pulses travel is water, with a GVD
parameter .beta..sub.2 of 8.times.10.sup.-28 s.sup.2/cm, a Kerr
index n.sub.2 of 4.5.times.10.sup.-16 cm.sup.2/W, a linear index
n.sub.0 of 1.3, and an absorption coefficient of .alpha.=0.1
m.sup.-1. FIG. 2 illustrates the intensity profile of the initial
pulse, and FIG. 3 illustrates the intensity profile of the pulse
after propagating through a distance of 11.4 meters. FIG. 3 shows
the extreme transverse self-compression caused by the nonlinear
self-focusing effect, producing an intensity level several orders
of magnitude increased from the initial level.
[0051] FIGS. 4 and 5 illustrate the amount of pulse compression
when propagating for a distance twice the attenuation length. The
initial optical pulse has a wavelength of 400 nm, an initial pulse
duration T(0) of 200 picoseconds, an initial pulse energy E(0) of
2.2 mJ, an initial power level P(0) of 40 P.sub.nsf, an initial
beam radius R(0) of 0.43 cm, and an initial noise amplitude of 10%.
The medium through which the optical pulses travel is water, with a
GVD parameter .beta..sub.2 of 8.times.10.sup.-28 s.sup.2/cm, a Kerr
index n.sub.2 of 4.5.times.10-16 cm.sup.2/W, a linear index n.sub.0
of 1.3, and an absorption coefficient of .alpha.=0.1 m.sup.-1. FIG.
4 illustrates the intensity profile of the initial optical pulse,
and FIG. 5 illustrates the intensity profile of the pulse after
propagating through a distance of 21.3 meters.
[0052] When the intensity of the optical pulse increases
sufficiently to cause laser induced breakdown in the liquid, the
liquid in a small region of high intensity ionizes. A threshold
intensity for laser induced breakdown (LIB), I.sub.LIB, is a
function of pulse length and wavelength. In water at visible
wavelengths, for a pulse length of 1 picosecond, I.sub.LIB is
experimentally determined to be in the range of 10.sup.11 to
10.sup.12 W/cm.sup.2, depending on wavelength and measurement
technique. Although not wishing to be bound by theory, it is noted
for clarity that laser induced breakdown can have at least two
mechanisms. One mechanism is multi-photon ionization by intense
illumination, and is the prevailing ionization mechanism for laser
pulses shorter than approximately 100 femtoseconds. A second
additional, slower mechanism is avalanche ionization for
significantly longer laser pulses. Avalanche ionization consists of
laser excitation of a small number of "seed" free electrons,
followed by collisional ionization by these electrons.
[0053] When the initial beam size is large and the initial power is
sufficiently high, longitudinal compression alone can be enough to
raise the intensity level of the pulse to I.sub.LIB without
significant transverse compression.
[0054] For monochromatic light, GVD does not play a role and only
NSF-induced transverse focusing will occur for powers above
P.sub.NSF. As discussed above, when the intensity reaches
I.sub.LIB, ionization will produce an acoustic pulse.
[0055] Following ionization, the plasma formed by ionization
strongly absorbs laser pulse energy, causing rapid phase change (to
vapor/plasma) and heating of the ionized volume. This heating
occurs on laser pulse time scales, which are extremely short
compared to acoustic transit times, so little or no significant
expansion of the superheated vapor occurs during the laser
pulse.
[0056] FIGS. 6 and 7 illustrate computer simulations showing the
effect of pulse compression on the pulse duration, spot size, and
pulse intensity. In this simulation, the initial optical pulse has
a wavelength of 400 nm, an initial pulse duration T(0) of 100
picoseconds, an initial pulse energy E(0) of 1 mJ, an initial power
level P(0) of 40 P.sub.nsf, an initial beam radius R(0) of 0.33 cm,
frequency bandwidth|.delta..omega./.omega.| of 2.5%, and negative
chirp. The water has a GVD parameter .beta..sub.2 of
8.times.10.sup.-28 s.sup.2/cm, a Kerr index n.sub.2 of
4.5.times.10-16 cm.sup.2/W, a linear index n.sub.0 of 1.3, and an
absorption coefficient of .alpha.=0.1 m.sup.-1. The corresponding
P.sub.nsf for 400 nm wavelength is approximately equal to 0.42
MW.
[0057] After passage of the intense electric field of the laser
pulse, the ionized volume begins to recombine, leading to the
optical emission of line spectra, in a relatively uniform
distribution in direction. As the laser pulse is propagating
through this ionized volume, it also results in self-phase
modulation and white light generation, the resulting photons of
which are primarily directed in the "forward" and "backward"
directions, in a cone around the original propagation direction of
the originating laser pulse. This will result in a broad spectrum
of light being sent in a cone around the original pulse direction.
White light can also be generated without sizable
plasma-generation, and the original wavelength (as well as the
other wavelengths present) can also be scattered/refracted into a
conical pattern around the originating laser pulse direction. The
resulting optical signature from the ionized volume is a
combination of relatively omnidirectional line spectra, as well as
a cone of white light propagating in a cone around the "forward"
and "backward" directions, and roughly the laser frequency
propagating in a cone in the "forward direction", both coherently
and incoherently, and with different distributions of polarization.
This optical signature is the illumination source we consider to
operate in conjunction with the generated acoustic source.
Following the rapid heating of the ionized volume, supersonic
expansion and shock generation occurs more slowly, at an acoustic
transit time .tau..sub.s approximately equal to d/v.sub.s, where
v.sub.s is the shock speed and d is the size of the ionized volume.
For typical laser energies, initial shock speed can be a few
multiples of the acoustic velocity in the liquid.
[0058] The acoustic pulse length of the generated acoustic pulse
can be determined by the acoustic transit time across the ionized
volume in the direction of sound propagation, for a pulse that is a
superposition of shock fronts generated from each initial point of
supersonic expansion. Thus, larger ionized volumes, and the higher
laser pulse energies required to produce them, produce longer
acoustic pulses. Embodiments of the invention also include a method
of controlling the duration of the acoustic pulse that accompanies
the illuminating optical source by tailoring the size of the
ionized volume through variation of the laser pulse energy.
Additional facets of specific embodiments call for control of the
optical illumination source in spectrum, direction, duration, and
polarization by controlling the input laser pulse parameters
discussed above.
[0059] Note that the acoustic pulse length is not necessarily the
same in all directions of acoustic propagation. Embodiments of the
invention include a step of adjusting the acoustic pulse by
tailoring the shape of the ionized volume. For example, a laser
pulse can be launched in which only GVD-induced longitudinal
compression to LIB intensity occurs, thereby producing a
disc-shaped ionized volume. This can produce longer acoustic pulse
lengths in acoustic propagation directions parallel to the plane of
the disc. Alternatively, for applications requiring only short
underwater laser propagation distances without LIB range
reproducibility, optical pulses with little or no frequency chirp
can be generated that rely only on nonlinear self focusing effects
to bring the pulse to LIB intensities.
[0060] When the laser wavelengths are in the range of 300-550 nm,
acoustic generation can be accomplished remotely by underwater
laser pulse propagation through distances up to or greater than the
attenuation length (up to tens of meters in seawater). In contrast,
when laser wavelengths are in the infrared range of about 1-11
microns, acoustic generation is confined to distances a few
centimeters from the laser source. Laser induced breakdown,
vaporization of the liquid, and shock generation for laser acoustic
generation is also more efficient by several orders of magnitude
than photo-acoustic generation via laser heating and thermal
expansion of water.
[0061] The laser 10 used to generate the optical pulse can be
located in air or another gaseous medium, with the optical pulses
being transmitted for a distance in the air, and into the liquid
medium.
[0062] In another embodiment, the laser 10 can be located in the
liquid itself, with the optical pulses being transmitted through a
window into the liquid. It is not necessary for the optical pulses
to be generated and propagated any distance in air before being
transmitted into the liquid.
[0063] Embodiments of the invention are also directed to
acoustic/optical-illumination generation systems having
applications in surgery, medical imaging, navigation, sonar,
communications, and countermeasures for acoustically-guided
undersea devices.
[0064] In an embodiment illustrated in FIG. 8, repetitively pulsed
laser 800 can generate optical pulses 810 that are steered by a
moving mirror or other steering mechanism 820. As the mirror
rotates, optical pulses steered along the arc generate
acoustic/optical illumination source pulses 830 in the desired
sequence and locations. These acoustic pulses can form a large
acoustic aperture sonar source for high resolution acoustic imaging
and multistatic acoustic scattering. The acoustic sources can be
generated at a high pulse rate and timed and positioned so they
form an acoustic phase front of a large aperture acoustic pulse.
The simultaneous optical illumination can be used to optically
image the object of interest, and once the location is known, the
mirror can be held stationary to point the laser pulse in the
direction of the object of interest. The optics train and laser
parameters can then be adjusted to position the acoustic/optical
source in the direction of the object of interest, either holding
the spot steady or moving toward and/or away from the object of
interest. The return optical signals can be spectrally decomposed,
temporally gated, and/or compared between different polarization
states and/or differing wavelengths (where in one embodiment, the
different polarization states or different wavelength returns are
subtracted from one another).
[0065] As an example, FIG. 9 illustrates a system in which a laser
910 and acoustic/optical detector 920 are on an underwater
platform, possibly tethered to a manned or unmanned surface ship
(and/or underwater vehicle and/or stationary structure) 900. The
laser generates a series of optical pulses 930, 940, 950, 960, 970,
which in turn compress and generate acoustic/optical illumination
sources/pulses. These acoustic and optical illumination source
pulses propagate and are reflected by the object of interest 980.
The acoustic/optical detector receives the reflected
acoustic/optical signals from the object of interest. Because the
locations of the optical pulses generated by the laser are known
based on the chosen laser pulse compression range and steering
mechanism setting, the system accurately determines position and
reconstitutes an image of the target. The acoustic/optical
detectors and/or laser can also be located on an undersea vehicle
not tethered to a surface ship or a on a stationary undersea
device.
[0066] Another embodiment is directed to a countermeasures system
in which the acoustic/optical pulses are generated so they
replicate an acoustic/visual signature of different mechanical
systems of interest or to disguise the true signature of an asset
to be masked.
[0067] Another embodiment is directed to a navigation system useful
for accurate identification of the position of an undersea vehicle,
and is illustrated in FIG. 10. Note that GPS is not available
without an in-air antenna, so underwater vehicles can have
difficulty maintaining accurate position information during lengthy
underwater transits. One or more acoustic/optical signals 150, 160,
and 170 are generated by a laser 180 carried by a surface ship,
aircraft, or satellite at prearranged locations and timings. The
AUV 100 receives the acoustic pings, and can identify its position
by triangulation, analogously to a GPS device triangulating via GPS
radio signals.
[0068] Regarding the optical imaging aspect of the invention, there
are a number of techniques that can be fruitfully employed to yield
important and accurate information about the object of
interest.
[0069] The approach we describe places a source of broadband
optical and acoustic energy at any desired location along a line of
sight, including directly on a target. This broadband energy will
come from the self-focusing and self-compression of an ultrashort
laser pulse in the water, culminating in the conical emission of
broadband "white" light, as well as an acoustic "snap" from the
rapidly-heated medium.
[0070] Pressure (p) can be calculated by using the approximate
relationship p=P.sub.ov(v-v.sub.o)/(2.1), where v is the shock
speed, v.sub.o is the sound speed in water, and P.sub.o. is the
water density. This can be used to determine the average pressure
from the average expansion velocity of the bubble.
[0071] The amount of deposited energy can be tailored to the size
and distance of the target, and can be very small since the source
of white light and acoustic energy can be positioned close to the
object of interest. White light generation is currently being
performed in air to achieve similar objectives of imaging and
spectroscopic remote sensing, and the disclosed technology employs
an extension of this technology to underwater applications. Optical
absorption is much smaller in the air, and this has allowed the
phenomena of interest to be controlled over distances of tens of
kilometers. Shorter ranges are anticipated under water.
[0072] The white light generated by the self-focused ultrashort
laser pulse has been used in the air to identify different chemical
species. In air, the white light illumination source can be formed
several kilometers away to measure the spectrum of the returned
signal back near the laser, after the white light is absorbed
through the atmosphere on the way back to the point of origin. This
same spectroscopic technique is anticipated to be useful under
water over many meters to allow spectral identification of
different compounds without a complicated and time-consuming
sampling technique. This is anticipated to prove successful because
of the very different spectra of most compounds of interest from
that of water. Typically, aqueous solution can also hold much
greater concentrations of an impurity than can air, resulting in
yet stronger absorption of the returned signal (and a stronger
spectral signature).
[0073] A nice broad spectrum of white light can be generated when
an ultrashort pulse propagates through water. The generated
spectrum can be broken up into a number of different frequency
bands to enable a variety of imaging and sensing techniques, using
very short and very intense white light interrogation pulses, which
can be generated at points along the direction of propagation
determined by the operator's choice of laser parameters.
[0074] Reflected pulses can be employed using different wavelength
filters (recall the broad bandwidth of a very short pulse) and
different polarization filters, as well as two different gate
times. The pulse shapes at different wavelengths and polarizations
are in general very reproducible, indicating that the pulses will
preserve the image fidelity, and that the different images (e.g.
from different polarizations, wavelengths, and time-gates) can be
linearly combined (added/subtracted) to extract with great accuracy
the otherwise-occluded details of the true image.
[0075] One of the most straight-forward methods to extract images
is to time-gate a single camera to preferentially capture the
photons that have enough time to propagate to the target and return
to the measurement platform without scattering. These are called
the ballistic photons, and an image captured within a short
time-gate that contains the ballistic photons can generate an image
of the target that is orders of magnitude stronger than an image
that captures all of the scattered light.
[0076] Beyond this, a number of other techniques can be implemented
when using more than one camera, including but not limited to the
dual-image subtractions listed below, which have been performed to
extract information, including enhancing/extracting images from
otherwise murky/occluded backgrounds. Potentially interesting
examples that employ dual-camera (or multiple camera) applications
include: [0077] 1. Synchronized cross-polarization imaging and
sensing (simultaneous spatial and temporal imaging and sensing in
perpendicular polarization states) can be used to enhance
materials-characterization and image-resolution. This works because
the diffusely scattered light is typically polarized differently
from the ballistic photons. [0078] 2. Self-calibrating fluorescence
lifetime measurements can also be obtained by scanning one of the
cameras in time to determine the length of a given fluorescence.
[0079] a. Camera-1 fixed I.sub.F(t.sub.o) [0080] b. Camera-2 varies
I.sub.F(t.sub.o+.DELTA.t) [0081] 3. Synchronized bi-spectral
imagery and sensing (simultaneous spatial and temporal imaging in 2
distinct spectral bands) can extract portions of the spectrum that
are preferentially reflected by the target to strongly enhance the
target-image. On very fast time-scales, this technique can also be
used to determine chemical reaction rates.
[0082] Underwater laser induced breakdown spectroscopy has already
been demonstrated using short pulse lasers to identify a large
number of elements, including Li, Na, K, Ca, Mn, and Zn at
pressures up to 272atm. Double-pulses were shown to be particularly
effective (Michel et al.). This added discrimination capability
using a UPL source is a powerful diagnostic when determining the
physical constitution and nature of an underwater object. Once an
object of interest is acoustically located, then it can be
interrogated using the underwater LIBS technique, using one or more
pulses at each point for the best results.
[0083] Beyond the optical methods described above, broad,
high-frequency acoustic interrogation will further aid in
identifying a material in question. Dolphins identify materials by
bouncing broad acoustic signals, centered around roughly 180 kHz,
off of their targets and listening to the echoes. We anticipate a
greatly enhanced discrimination ability using the much higher and
much broader acoustic signature produced with the ultrashort pulse
laser cavitation. This serves as an extension of the
acoustic-identification phenomena already used in nature, and can
be employed in a variety of applications.
[0084] In air, spectroscopy has been performed using the
backscattered white light to measure various atmospheric
constituents. This technique can be extended to underwater
environmental sampling. We foresee its utility in determining the
presence of trace explosives, combustion products, pollutants, and
hazardous materials.
[0085] The technique will result in improved acoustic and optical
fidelity with which targets can be resolved, using much lower and
more localized power requirements. We anticipate a sufficient
concentration of acoustic and optical energy on target, and very
little not on target. Improved speed with which high-fidelity
images and spectra can be obtained of the target, including
spectral characterization of a water-volume of interest. This is
anticipated to be nearly instantaneous, affording immediate
identification of target material and hazardous chemicals/materials
in the water. Important targets and/or materials can also be
identified through turbid water, demonstrating a new and valuable
capability.
[0086] The disclosed invention extends the UPL remote-sensing
techniques already being contemplated in air, and will expedite and
improve both qualitative and quantitative characterization of
underwater objects and chemicals present in the ocean environment.
Chemical analyses which can currently require hours or days will be
performed spectrally, allowing for immediate identification of
hazards and response to remediate them. Regarding underwater
objects of interest, the disclosed invention will dramatically
increase both the accuracy and speed of target identification.
Knowing that a target is made of wood, steel, or Aluminum (or any
other constitution) will result in a much faster determination of
how to deal with it. Knowing that the surrounding water does or
does not contain trace concentrations of explosives, fuel, or other
chemicals or hazardous material will also dramatically increase the
accuracy of how to deal with the target of interest. This spectral
information, coupled with the accompanying high-fidelity
multi/hyper-spectral and acoustic images will allow for faster and
more accurate responses, as well as far less frequent
categorization of objects as "unknown", relegated to further
investigation in the unknown future. In addition to benefiting
current Government operations, this ability will also help in the
identification and removal of unexploded ordnance and decrease the
acoustic and optical energy required in such characterization,
thereby reducing the environmental effect of these operations.
[0087] There are a number of areas that can strongly benefit from
this technique, including aquaculture enterprises, which have an
extremely strong interest in real-time tracking of the constitution
of their water column. This is necessary to maintain quality
control and to abide by Government regulations. These capabilities
will be helpful anywhere sampling is currently required, such as
waste-water management and recreational beach usage. As the world's
population continues to grow, these areas are becoming increasingly
problematic and will be able to benefit from real-time, remote,
non-invasive sampling capabilities.
[0088] We envision a system that will allow immediate optical,
acoustic, and spectral characterization of objects of interest, as
well as immediate spectral characterization of trace compounds in
the water. These applications are of great importance with respect
to: swimmer detection; unexploded ordnance; mapping the ocean
terrain; identifying vehicles (keeping in mind regular exercises
for undersea warfare and other war games); Maritime Domain
Awareness; efforts employing multiple cameras to build on the
single-camera applications/capabilities, including
comparison/subtraction of images filtered with different polarizers
and/or spectral filters.
[0089] Many of these techniques pertain directly to medical imaging
which is also a preferred embodiment. The body structures are made
up of: soft tissue resembling water; air in the stomach, lungs, and
intestines; and hard tissues, such as bone. Biological tissues can
also be probed/imaged both optically and acoustically using the
disclosed invention. The longer wavelengths (IR) are typically best
for optically penetrating the soft body tissues. In this case, the
body can be optically probed and mapped to identify locations that
require attention, possibly from acoustic energy or LIBS. The
optical probing can again take place, temporally gating the pulse
returns to eliminate scatter and/or also comparing/differencing
different polarization states and/or spectral windows. Conversely,
as with the underwater case, the body can first be mapped
acoustically to identify areas of interest and then optically
mapped, based on the acoustic guidance.
[0090] Candidate Claims will include, but are not limited to
combinations of the various diagnostic capabilities afforded by the
ultrashort pulse laser interactions with the liquid, including: the
high-frequency, broad-spectrum acoustic signature that comes from
the rapid expansion of the vaporized liquid; the laser-induced
breakdown spectrum of the liquid itself and of a surface of
interest; the broad-spectrum, conically-directed white-light
generated by the laser focus; time-gated imaging of the target,
whose distance can be determined by the acoustic return; spectral
comparison using filters and the broadband illumination;
polarimetric comparison, using the differences in polarization of
the ballistic photons from the scattered ones.
[0091] Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that the claimed invention may be
practiced otherwise than as specifically described.
[0092] Another embodiment includes a focusing lens near the laser,
where the optical pulse begins its underwater propagation. Initial
optical pulse intensity is limited by filamentation instabilities.
The lens can serve to collect and transversely focus more pulse
energy than would otherwise be possible given this intensity limit
and the collimated beam size required for non-linear transverse
self-focusing at a given distance.
[0093] The invention has been described with reference to certain
preferred embodiments. It will be understood, however, that the
invention is not limited to the preferred embodiments discussed
above, and that modification and variations are possible within the
scope of the appended claims.
* * * * *